Pilot schemes for the plans found that free school meals for all children “led to significant increases in attainment”, with key stage two children found to be academically two months ahead of their peers.

The review - commissioned by the Department for Education and the Department of Health - suggested this could be because the children were having a healthy meal and were being forced to socialise together.

It said: “Potential explanations for such effects may include the benefits of having a nutritious meal at lunchtime, the social benefits of children eating a meal together and potentially more positive relationships between parents and the school.”

The review described how children demonstrated “improved social skills at meal times” and how the researchers saw “a ‘levelling effect’ for children in universal areas who opted for a free school meal”.

The review said that offering the free meals to all primary school children reduced the “socio-economic differences in the quality of food eaten at lunchtime” because poorer children were no longer eating cheap packed lunches.

It added: “This finding provides some suggestive evidence that universal entitlement to free school meals may contribute towards reducing the gap in attainment between pupils from different socio-economic backgrounds.”

The pilot was also seen to have improved the social skills and table manners of pupils by virtue of encouraging pupils to sit together for meals.

One parent said: “They learn to talk to each other, listen as well to each other … it’s like getting together … they’re not learning from the school, they’re learning from each other.”

Mr Clegg told The Telegraph: “The evidence is clear - free school meals will not only save families hundreds of pounds a year but will also have a dramatic impact on how a child performs works in the classroom so that, regardless of their background, every child can have the best possible start in life.

“This is one of the most progressive changes to our school system for a long time. There will always be critics, but that won’t cloud my goal to create a level playing field for all of our children so their success will be determined by their talents and efforts alone and not by their parents’ bank balance.”

Around 400,000 children in the first three years of primary school are currently eligible for free school meals. The change will mean another 1.5 million children no longer have to pay for lunches.